The present invention relates to a drive force distribution device and a method for distributing drive force.
A typical drive force distribution device is capable of changing a drive force distribution ratio of a main drive wheel to a sub drive wheel. Typically, a torque coupling is provided in a drive force transmission system. Operation of the torque coupling is controlled in such a manner as to change the torque transmission rate (the transmitted torque) from an input to an outlet of the torque coupling. This regulates the drive force distribution ratio of the main drive wheel to the sub drive wheel.
The torque coupling employs a friction clutch, which generates heat through frictional engagement of clutch plates. Also, a transfer case or a differential, which are arranged in the drive force transmission system of a vehicle, generate heat through friction caused by engagement of gears. Overheating of these heat generating portions thus must be suppressed.
For example, Japanese Laid-Open Patent Publication No. 2003-136990 describes a drive force distribution device that detects the temperatures of a differential or a transfer case provided in a drive force transmission system. If the temperature of the differential or the transfer case exceeds a corresponding predetermined level, the drive force distribution device controls operation of a torque coupling in such a manner as to suppress overheating of the differential or the transfer case.
Alternatively, Japanese Laid-Open Patent Publication No. 7-12155 describes a method for estimating the temperature of a torque coupling in correspondence with the rotational speeds (difference of the rotational speeds) of an input shaft and an output shaft provided in the torque coupling, torque transmitted by the torque coupling, and the temperature of hydraulic fluid supplied to the torque coupling.
However, to provide temperature sensors in each of the heat generating portions, an increased cost is needed to prepare, assemble, and wire the parts. Further, to estimate the temperature of each heat generating portion based on the temperature of the hydraulic fluid supplied to the heat generating portion, the vehicle must have fluid temperature sensors that can detect the fluid temperatures in the heat generating portions.